The objective is to analyze factors governing tolerance and autoreactivity among murine CD4+ T and B cells in transgenic mice that express the influenza virus PR8 hemagglutinin (HA) as a well-characterized neo-self antigen (HA Tg mice). In particular, how the distinct specificities of separate populations of CD4+ T and B cells affect their negative selection by the neo-self HA, and processes that can lead to the activation of autoreactive lymphocytes in HA Tg mice will be examined. Aim 1 will examine the selection and functional potential of autoreactive CD4+ T cells that have low avidities for a self-peptide in TCRxHA Tg mice. Whether activation increases the sensitivity of low avidity T cells to the extent that they become responsive to a self-peptide will be determined. In addition, how TCR specificity and/or virus infection contributes to the ability of CD4+ T cells to mediate autoimmune myocarditis in HA Tg mice expressing the HA in cardiac tissue will be assessed. Aim 2 will examine factors governing the phenotypic potentials of B cells that express characteristic variable region clonotypes, that are representative of primary versus memory responses to the HA in virus-immunized BALB/c mice, and that differ in their sensitivity to negative selection in HA Tg mice. Whether selection into different B cell subsets and/or affinity for the HA determines the distinct phenotypic potentials of B cells expressing these clonotypes will be evaluated. In addition, whether autoreactive CD4+ T cells rescue primary response B cells from deletion and/or promote memory B cell formation in response to the neo-self HA will be assessed. Aim 3 will examine the processes that lead to the development of organ-specific autoimmunity. An autoimmune syndrome resembling rheumatoid arthritis develops in TCRxHACII mice in which the HA is expressed on antigen presenting cells, and the cellular interactions that lead to its development will be assessed. Whether virus infection provokes autoimmunity in TCRxHACII mice expressing low affinity CD4+ T cells and/or CD4+ T cells directed to a cryptic self-peptide will also be examined. These studies will provide fundamental insights into the mechanisms of immune tolerance, and will have direct relevance to understanding the processes that lead to the development of autoimmune disease.